1. Field of the Invention
The present invention pertains to the field of liquid precursor solutions that may be used to produce solid metal oxide materials having specialized electrical properties. More specifically, the preferred precursor solutions include an octane solvent mixed with polyoxyalkylated metal complexes of a type that may be used in liquid deposition processes for manufacturing thin-film electrical components and, especially, ferroelectric or dielectric materials for use in integrated circuits.
2. Statement of the Problem
The use of hazardous chemicals in manufacturing processes has commensurate environmental and civil liability risks; however, these chemicals often continue to be used because no suitable replacement can be found. Even where replacement chemicals can be employed, manufacturers often choose to continue use of the old chemicals because of perceived quality and reliability problems in changing the manufacturing process. For example, the commercially available xylene solvents are typically a mixture of ortho, para, and meta xylene/somers. As of 1975, xylene was the 26th highest-volume produced chemical in the United States, and large xylene quantities are still produced by fractional distillation from petroleum. Nevertheless, human health-safety concerns have led government agencies to establish a safe threshold limit value for workplace exposure to xylene at about 100 ppm. Xylene, which typically has a flash point ranging from about 27.2 to 46.1.degree. C. (values may vary depending upon the grade of xylene), also constitutes a dangerous fire hazard. Similar problems exist with the use of alcohols, ethers, esters and ketones, as well as with other aromatic hydrocarbons in addition to xylene.
Metal oxide films for use in integrated circuits have most frequently been formed by conventional sputtering techniques. See for example, Kuniaki Koyama, et al., "A Stacked Capacitor With (Ba.sub.x Sr.sub.1-x)TiO.sub.3 For 256M DRAM" in IDEM (International Electron Devices Meeting) Technical Digest, December 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. Other fabrication methods include pulsed laser deposition, and rapid quenching as listed in Joshi, P. C. et al., "Structural and Optical Properties of Ferroelectric Thin Films By Sol-gel Technique," Appl. Phys. Lett., Vol 59, No. 10, November 1991. These methods are relatively violent processes and, thus, inherently result in relatively poor control of the composition of the final thin film as a whole and variable composition throughout the film.
Metal oxide films have also been formed from sol-gels, i.e., a metal alkoxide which is hydrated to form a gel. These gels are applied to a semiconductor substrate to form a film, and then decomposed to form a metal oxide. One such method comprises the application of a sol-gel to a substrate followed by heat treatment. The heat decomposes the sol-gel and drives off the organics to form the metal oxide. See for example, U.S. Pat. No. 5,028,455 issued to William D. Miller et al., the Joshi article cited above, and B. M. Melnick, et al., "Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT for Ferroelectric Memories", in Ferroelectrics, Vol 109, pp. 1-23 (1990). In another method, what has been termed a "MOD" solution is applied to a substrate followed by heating which decomposes the MOD solution and drives off the organics to form the metal oxide. See "Synthesis of Metallo-organic Compounds for MOD Powers and Films", G. M. Vest and S. Singaram, Materials Research Society Symposium Proceedings, Vol. 60, 1986 pp. 35-42 and "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", J. V. Mantese, A. L. Micheli, A. H. Hamdi, and R. W. Vest, in MRS Bulletin, October 1989, pp. 48-53. Generally the sol-gel method utilizes metal alkoxides as the initial precursors, while the MOD technique utilizes metal carboxylates as the initial precursors. These techniques require the addition of water to the solution prior to application of the solution to a substrate. The use of water induces undesirable chemical reactions, e.g., the possible precipitation of metalized reagents and severe viscosity changes.
The above references typically discuss precursor compounds having metals that bond with organic ligands. These ligands must be broken down and removed during the heating-decomposition process. The molecular geometry creates relatively large distances across which the metal and oxygen atoms must link to form metal oxides. These distances can often result in cracking or other imperfections in the film, and, accordingly, impose a severely burdensome manufacturing duty of exacting control over multiple parameters, such as film thickness, drying and annealing temperatures, the substrate used etc.
In other liquid deposition processes, such as the sol-gel process described in the Melnick reference, the metal-oxygen-metal bonds of the final metal oxide are present in some degree; however the precursor is highly unstable and, therefore, is unsuited for use except immediately after preparation in the laboratory. One sol-gel reference, the Miller patent referenced above, mentions one metal carboxylate, lead tetra-ethylhexanoate, as a possible precursor; however the reference fails to disclose how this substance may be used as a sol-gel, and, furthermore, rejects this precursor as being less desirable because the large organic group was thought to result in more defects in the final film.
Thin-film metal oxide electronic components, i.e., those having thicknesses of less than about ten microns, may often require ferroelectric or dielectric properties. The film should have a relatively uniform grain size, which results in better crystalline qualities such as films free of cracks and other defects. The film grain size should also be small compared to the thickness of the film; otherwise the roughness of the film can be comparable to the thickness and other dimensions of the device components, which can make it difficult or impossible to fabricate devices within performance tolerances and can result in short circuits or other electrical breakdowns. Further, it is important that the fabrication processes be performed relatively rapidly, since long processes are more expensive in terms of the use of facilities and personnel.
In integrated circuit construction, it is sometimes useful to employ materials that exhibit relatively strong ferroelectric and dielectric behavior. These materials may include perovskites, and especially ABO.sub.3 perovskites, such as barium titanate, wherein A and B are respective A and B site metal cations. In addition to having ferroelectric properties, the perovskite-like layered superlattice materials discovered by G. A. Smolenskii, V. A. Isupov, and A. I. Agranovskaya (See Chapter 15 of the book, Ferroelectrics and Related Materials, ISSN 0275-9608, [V.3 of the series Ferroelectrics and Related Phenomena, 1984] edited by G. A. Smolenskii (especially sections 15.3-15), may also have high dielectric constants. On the other hand, these types of materials are not widely used on a commercial basis due to problems with polarization fatigue and retention of the ferroelectric polarization state. These problems are thought to result from uncompensated defects in the ferroelectric crystalline structure and associated ionic charge migrations.
Integrated circuits, which are sometimes called semiconductor devices, are generally mass produced by fabricating hundreds of identical circuit patterns on a single wafer. This wafer is subsequently sawed into hundreds of identical dies or chips. While integrated circuits are commonly referred to as "semiconductor devices" they are in fact fabricated from various materials which are either electrically conductive, electrically non-conductive, or electrically semiconductive.
The material out of which the wafer and other parts of integrated circuits are fabricated is generally either silicon (Si) or gallium arsenide (GaAs). Silicon is the most commonly used material, and the present invention will be described in terms of silicon technology. Nevertheless, the invention is also applicable to semiconductor technologies based on GaAs or even other semiconductors. Silicon can be used in either the single crystal or polycrystalline form in integrated circuits. In the integrated circuit fabrication art, polycrystalline silicon is usually called "polysilicon" or simply "poly", and will be referred to as such herein. Both forms of silicon may be made conductive by the addition of impurities, which are commonly referred to as "dopants." If the dopant is an element such as boron which has one less valence electron than silicon, electron "holes" become the dominant charge carrier and the doped silicon is referred to as p-type silicon. If the doping is with an element such as phosphorus which has one more valence electron than silicon, additional electrons become the dominant charge carriers and the doped silicon is referred to as n-type silicon.
Silicon dioxide is commonly used as an insulator or barrier layer in silicon-based semiconductors devices. Its use is so universal that in the integrated circuit art it is often referred to as simply as "oxide". Another common silicon-based structure is called polycide. This is a composite, layered material comprising a layer of metal silicide and a layer of polysilicon. CMOS (Complimentary Metal Oxide Semiconductor) technology is currently the most commonly used integrated circuit technology, and thus the present invention will be described in terms of silicon-based CMOS technology, although it is evident that the invention may be utilized in other integrated circuit technologies.